The control of the exothermic oxidation reactions of solid materials is a real problem notably in the nuclear field, in particular in the context of processes for oxidizing nuclear fuels in the form of carbides or nitrides containing plutonium and/or uranium, before storage or stock-piling or before reprocessing by nitric dissolution, or even in the context of management of manufacturing waste produced in the cycle for producing these fuels. These unoxidized, nitride or carbide, nuclear fuels are all highly reactive with oxygen and potentially pyrophoric. Transforming them into oxides may obviate certain chemical risks.
Plutonium-containing carbide fuels (U,Pu)C have a very high affinity for oxygen, which may affect the stability of the product during various manufacturing operations, and the oxidation reaction of these fuels may lead to a highly exothermic thermal runaway according to the following highly exothermic (ΔrH≈−1250 kJ/mol) reaction:MC+O2→MO2+M3O8+CO2, where M=U, Pu.
For reasons of safety the reaction must be controlled at all times.
Pure or alloyed metallic plutonium may be the material concerned in these oxidation processes. A container allowing the oxidation of plutonium-containing metals to be controlled has already been described in French patent application FR 2 752 234. More precisely, this prior-art container is a multilayer container specifically designed to contain a reactive system that is liable to melt. The confinement chamber consists of at least one sheet made of a first material chosen from tantalum, tungsten and their alloys, this sheet being inserted between at least two sheets made of a second material that does not react with air at the temperatures of the process used; this second material may advantageously be stainless steel.
During the oxidation reaction, this multilayer chamber undergoes very little oxidation and if the plutonium melts and then accidentally spills, and the stainless steel is pierced, the tantalum retains the plutonium, and consequently the chamber produced is safe.
The same type of problem may also be encountered in fields such as the construction field or the field of industrial buildings, for example in functionalized and potentially active roofs or ceilings for warehouses or workshops or cabinets in which very inflammable chemical materials or fuels are stored or worked on.
Generally, in process management, potential runaway of a chemical reaction associated with—or a consequence of—substantial overheating is conventionally managed, manually or automatically, using a set of sensors and feedback. This type of management, although it is effective, firstly relies on a transfer of information from the reactor in which the reactive system is found to the one or more sensors in question, and then to the control/command module, so that in return one or more expected actions can be carried out. This information flow and its management, ensuring the corrective action is implemented, cannot be instantaneous. They also depend on the sensors and all the elements in the information chain operating correctly. Finally, these processes involve an electrical power supply, which even if it is advantageously separate from the power supply nominally used for the process, must be reliable and available, similarly to the system provided a priori for fail-safe management of the neutron reactivity of a nuclear reactor core in the case of runaway, namely control bars and the system allowing them to drop very rapidly so as to absorb the neutrons.